Modified function clean_filename of Plugin Clean Image Filenames by @Upperdog

clean-filename-function.php

<?php 
/**
 * Clean filename.
 *
 * Performs the cleaning of filenames. It replaces whitespaces, some specific
 * chacters and uses WordPress' remove_accents() function for the rest. It
 * also converts filenames to lowercase.
 *
 * With a little tweak based on a hint by @Zodiac1978:
 *
 * @link https://core.trac.wordpress.org/ticket/30130
 *
 * @since 1.3 Rewrote filename cleaning to better handle specific characters.
 * @since 1.1 Added function.
 *
 * @param array $file Uploaded file details.
 * @return array $file Uploaded file details with cleaned filename.
 */
public function clean_filename( $file ) {

	// Get file details.
	$file_pathinfo = pathinfo( $file['name'] );

	// Get filename without extension.
	$cleaned_filename = $file_pathinfo['filename'];

	// Replace whitespaces with dashes.
	$cleaned_filename = str_replace( ' ', '-', $cleaned_filename );

	// @Zodiac1978's tweak
	if ( function_exists( 'normalizer_normalize' ) ) {
		$cleaned_filename = normalizer_normalize( $cleaned_filename, Normalizer::FORM_C );
	}

	// Perform WordPress remove accents.
	$cleaned_filename = remove_accents( $cleaned_filename );

	// Convert filename to lowercase.
	$cleaned_filename = strtolower( $cleaned_filename );

	// Remove characters that are not a-z, 0-9, or - (dash).
	$cleaned_filename = preg_replace( '/[^a-z0-9-]/', '', $cleaned_filename );

	// Remove multiple dashes in a row.
	$cleaned_filename = preg_replace( '/-+/', '-', $cleaned_filename );

	// Trim potential leftover dashes at each end of filename.
	$cleaned_filename = trim( $cleaned_filename, '-' );

	// Replace original filename with cleaned filename.
	$file['name'] = $cleaned_filename . '.' . $file_pathinfo['extension'];

	return $file;
}